The invention relates to an X-ray camera apparatus having a function to switch a visual field of an X-ray image and, more particularly, to an X-ray camera apparatus which can obtain a good X-ray image even when the visual field of the X-ray image in an image intensifier is switched.
The X-ray TV camera apparatus is applied to an image diagnosing apparatus to obtain a fluoroscopic image of an object to be examined and is an apparatus in which an X-ray image that is obtained by passing through the object to be examined is converted into a visible light by an image intensifier (X-ray fluorescent multiplier) and the visible light is converted into an electric signal by a TV camera.
The electric signal which is obtained by the TV camera is converted into a the digital signal and is subjected to proper image processes and, after that, the processed signal is supplied to a display apparatus such as a CRT or the like.
In the X-ray TV camera apparatus with such a construction, an amount of X-ray which is irradiated to the input surface of the image intensifier is not uniform. That is, a difference in uniformity occurs between the central portion and the peripheral portion since the input surface of the intensifier is curved, a lens of the TV camera is curved, and the like. Because of those causes, there occurs a phenomenon such that the electric signal from the TV camera decreases as the position of an optical image is deviated from the center (optical axis) of the optical image to the periphery. Therefore, correcting means for correcting such a phenomenon, namely, so called shading correcting means is provided. The shading correcting means is disclosed in "Image Electronic Circuit", issued by Corona Co., Ltd., pages 89 to 93 and 110, Oct. 15, 1980.
As disclosed in "Shimazu's Criticism", pages 113 to 119, issued on September, 1963, for example, as such an image intensifier, there has been known an image intensifier having a function in which a visual field of an optical image obtained by the image intensifier can be varied in accordance with the necessity.
In the X-ray TV camera apparatus with the above construction, however, the correction setting in the shading correcting means is unconditionally, namely, fixedly determined on the basis of the above causes. Therefore, even when the visual field of the image intensifier is switched, its correction amount is never changed.
In recent years, the size of screen of the image intensifier has been increasing more and more. A luminance difference between the central portion and the peripheral portion of the X-ray image, which doesn't a large problem even when the visual field of the X-ray image is changed in the conventional small image intensifier, increases in association with the increase in screen. Consequently, a uniform X-ray image cannot be obtained due to the size of visual field.
That is, in the case where the visual field of the image intensifier is switched, the image pickup area of the input surface of the image intensifier changes in accordance with the switching. The curved state of the input surface of the image intensifier of each image pickup area substantially changes (when it is optically observed) each time the visual field is switched. Thus, it has been found out that an attenuation amount of the electric signal from the TV camera changes.
Therefore, there is a problem such that the conventional shading correcting means cannot sufficiently correct because its correction amount is fixed.